In the case of a workpiece processed by a machine tool, generally, the criteria for evaluating the machined surface is surface accuracy, in particular surface roughness of the machined surfaces of the workpiece. Conventionally, the lower the surface roughness, the more it is regarded as good machining. However, recently, due to strips, caused by cusps formed on the machined surfaces, observed during a visual inspection by an observer or gritty-texture sensed by an observer when he touched the surfaces with his finger, there are times when the processed workpiece is rejected, even if the surface roughness of the processed workpiece is lower than acceptable by a user. For this reason, technologies have been proposed to measure the shiny appearance or the texture of a machined surface in addition to the simple measurement of the surface roughness. In this context, it should be noted that cusp means unprocessed micro portions on the machined surfaces of the workpiece or micro protrusions and concaves formed in the machined surfaces by vibrations of the machine tool induced by servo-control when a workpiece is cut with a rotating tool along a tool path including pick feeds or scraped with a spring-necked bit.
Patent Literature 1, for example, describes a method for evaluating surface property, relative to criteria of a plurality of measuring items, including specularity, by projecting a cross-stripes pattern, displayed on a display, to a surface to be measured, capturing the reflected image by an image pick up, and measuring the image data by a computer relative to specularity, the dispersion of the specularity, glossiness, sharpness, undulation or form error, contrast and opacity sequentially. Further, Patent Literature 1 describes, as a prior art, a method for measuring surface property by using a specular gloss meter for measuring the glossiness based on the amount of light received.
Further, Patent Literature 2 describes a method for measuring surface texture by combining maximum height of a shape (Rz), mean wave length of the shape (RSm) and surface texture parameter.    Patent Literature 1: Japanese Unexamined Patent Publication No. 2012-215486    Patent Literature 2: Japanese Unexamined Patent Publication No. 2003-500677